Diffraction will vary with the physical size of the opening and not the
focal ratio. A small diameter, short focal length lens as used in a
point and shoot digital might become diffraction limited at f/8. A much
larger medium format lens maybe not until f/32.
Theoretically, resolution is also a function of physical aperture and
not focal ratio. That's why big telescopes are better than little
telescopes. Apart from superior light gathering ability a large
diameter telescope resolves more detail than a smaller one. At least
until things like atmospheric turbulence begin to limit the resolution.
Lots of practical optical things get in the way of the theory but the
theory says bigger diameter is better for resolution.
A typical medium format lens would have much less resolving power (in
terms of lpmm) than a small, well-made point and shoot lens. But the
reason is not theory but that they are made to different standards. The
medium format lens doesn't have to have such high resolving power
because the image won't be magnified as much as the P&S image. Over a
larger area the MF lens will still resolve more detail.
Chuck Norcutt
jking@xxxxxxxxxxxxxx wrote:
>
> I have been spending some time thinking about sensor size (film or CCD).
> it seems to me that there is a counter argument to big is better and I
> think it goes somthing like this.
>
> Ok the laws if ohysics and particularly defraction limit resolution as the
> size of the hole the appature blades makes gets smaller. So for high
> resolution (assuming a perfect lens) to run at less than f8. Now the
> larger the sensor you have the higher the focal length you need to cover
> the same area as a small sensor for example by 6x4.5 bronica needs a 40mm
> lens to cover the olympus om4ti's 24mm lens. Since depth of field/focus
> decreases with focal length large sensors require higher appature number
> for the same depth of field and become defraction limited. To get the same
> depth of field out of my 40mm Bronica lens as my olympus 24mm at f8 I need
> f16 which according to Ziesses web site on theretic resolution vs appature
> is the difference between Therefore small sensors win because you can get
> the same depth of field as a larger sensor AND get less defraction.... at
> f8 ziess suggest 200lpmm is possible at f16 its half that.
>
> the table is taken from ziess'es web site
>
> f-no......................resolution
> .............................(line pairs per millimeter)
> .45.............................35
> .32.............................50
> .22.............................70
> .16...........................100
> .11...........................140
> ...8...........................200
> 5.6...........................280
> ...4...........................400
> 2.8...........................560
>
> Any comments?
>
> James
>
>
>
>
>
>
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